1. Field of the Invention
The present invention relates to a technology for conveying a printing sheet in an image forming apparatus.
2. Description of the Related Art
The final object of a typical image forming apparatus is to form a visible image on a printing sheet irrespective of whether the image forming apparatus employs the technique of electrophotography or ink-jet printing. For example, in an image forming apparatus that includes an automatic document feeder (ADF), the ADF feeds an original onto an original glass plate sheet by sheet, and an image reading unit reads an image of the original. After the image is read, the original is conveyed onto a document receiving tray of the ADF. On the other hand, a printing sheet stored in a paper feed unit of the image forming apparatus is conveyed to an image forming unit, where the image of the original is transferred and fixed onto the printing sheet, and then the printing sheet with the image formed on it is output. In these days, functions of the image forming apparatus have improved, and there is a need for increasing the speed of conveying printing sheets.
A typical paper feed unit includes a paper feed tray rotatably disposed so that a pressing unit such as a spring forces the topmost printing sheet in the paper feed tray toward a conveyance roller, and a separating unit separates one printing sheet from other printing sheets supplied by the conveyance roller from the paper feed tray. At this time, if a paper feed pressure applied by the paper feed tray on the topmost printing sheet is too high, the separating unit cannot separate one printing sheet, resulting in feeding of multiple printing sheets. On the contrary, if the paper feed pressure is too low, i.e. lower than a conveyance load of a paper feed guide or the like, the printing sheet is not supplied to the conveyance roller. In other words, it is necessary to set the paper feed pressure within a proper range.
Furthermore, to prevent a damage to the printing sheet and a misfeed due to an edge of the printing sheet being caught while it is conveyed from the paper feed unit to the separating unit, the printing sheets in the paper feed tray need to be near the separating unit or a paper feed guide unit of the paper feed unit, where the printing sheets may possibly contact the separating unit or the paper feed guide. Moreover, a guide unit that aligns a side and a tail of the printing sheets is provided to the paper feed unit to prevent the printing sheet from misalignment and skewing, and the guide unit also needs to physically contact the printing sheets. Sliding resistance by such physical contact causes a loss of the paper feed pressure. The proper range of the paper feed pressure needs to be determined in consideration of the loss. However, the amount of the loss is greatly affected by how a user feeds the printing sheets, and therefore the loss may be much larger than what is expected.
Technologies for reducing frequency of the misfeed by detecting a failure to convey the sheet and retrying to convey the sheet are disclosed in Japanese Patent Application Laid-open No. 2006-117405 and Japanese Patent Application Laid-open No. 2002-128323. However, because the printing sheet is conveyed again from the same point where the conveyance failed, and because static friction is higher than dynamic friction, there is even higher risk of a misfeed at the second time than the first time.
In another technology disclosed in Japanese Patent Application Laid-open No. 2007-183352, the frequency of the misfeed is reduced by intermittently conveying the printing sheets. However, because the force of conveyance after an interval needs to be higher than the static friction, there is also the risk of the misfeed.